I don’t believe he was referring to the Docetics as “the true Christians”, rather he was explaining their position on the crucifixion. In my opinion, the early Christians that held similar beliefs to Muslims about Jesus would have been the Jewish Christians led by James. They believed Jesus was a prophet and not deity.
If the Docetics are not “true Christians,” and therefore true Muslims (for Islam believes all true followers of God are simply Muslims), then he really has no point to be making. If they are not real Christians. If they are not real Christians, and therefore not real Muslims, then why bother with their opinion at all? Why look at people whom Christians and Muslims would both agree are heretics?
As for the true Christians being the “Jewish Christians” led by James, there is no evidence to support that he supported them or that they believed Jesus was just a prophet and not deity. This is especially true if you’re responding to the Judaizers in Acts: they believed in Christ’s deity but held the Law in higher regard than Grace and Truth; they also could not be led by James, as he speaks out against their theology at the Council of Jerusalem in Acts 15. The apostles worked as a group, they spoke as a group, and they believed as a group - and they all believed that Christ was deity. If you are referring to groups such as the Ebionites, there were actually various beliefs within their movement and they were condemned early on, especially by early Church Fathers such as St. Ignatius (who was a disciple of the apostle John).
you mentioned the Judas-Replacement-Theory and that Jesus was quoting psalms 22. I’m curious to know what is the biblical evidence to support this view. Because when I read the relevant section in Matthew, I didn’t get the impression he was quoting, rather I got the impression he was asking for help.
The best scriptural evidence for this is simply reading Psalm 22

Let’s review.
And about the ninth hour Jesus cried out with a loud voice, saying, “Eli, Eli, lema sabachthani?” that is, “
My God, my God, why have you forsaken me?” [Matt 27:46]
My God, my God, why have you forsaken me? Why are you so far from saving me, from the words of my groaning? [Psalm 22:1]
Psalm 22 is one giant Messianic prophesy. It foretells the isolation, mockery and execution of the Messiah, but ends with His glorification. This is not simply Byzantine_Wolf’s opinion, but has been the Church’s interpretation as well as Christian interpretation for some time. For example, Saint John Chrysostom makes reference to Psalm 22 in his exegesis of this passage, and proclaims it as the passage Jesus is citing. One finds Jesus quoting the psalms throughout His earthly ministry, therefore it is not strange that He would be quoting it here.
Now, why would He quote it, you might ask? A few reasons: 1) He is citing prophetic scripture to show who He is and what is truly happening; 2) He is showing sympathy to those who feel isolated from God - through His human nature Christ feels that isolation, and He cries out what an isolated person would feel to show His compassion towards those who in the past, present and future have felt it. To put it crudely, it was Christ letting those who have always felt isolated know that He, the Incarnate Word, knew what it felt. It was part of His suffering which He endured to purify our flesh and return us to the embrace of God.
You must also remember that to the Jews of that time this passage would have meant something, as they used the psalms in services and memorized scripture, and therefore would find it familiar. It would be the equivalent of me whispering before death, “Amazing grace…”. Those listening to me would know that I was referring to a popular hymn and not being astounded by a woman named Grace.

They would also know what that hymn was about, and put it into the context of what was happening.
Therefore when Christ calls out, “My God, My God, why have You forsaken Me?”, it makes perfect sense within the context of what is happening (especially when we remember everything that happens
before and
after it is made, within the gospel), and His purpose is sound.